I made some progress here.
Earlier I had already taken the HSDB2A unit to my friends house. He has a standard consumer router, nothing fancy. I then tried to install it at his place but it failed the same way as at did at my home.
However today I borrowed another standard router from another friend. To my surprise I got the HSDB2A to register on the first try.
I then deleted the device in the app (because that's the only way to change the WIFI SSID that HSDB2A connects throgh)
I tried again adding it to my ordinary WIFI. Now it worked all of a sudden!!! What has changed? Nothing in my network or the setup here for sure. Maybe you can try if you can reproduce that @Drekameger
Great News, when you say standard consumer router, what brand/model # worked?